This invention is directed to a microwave oven comprising a microwave source and a power unit including a high voltage transformer for supplying high voltage to said microwave source, said high voltage transformer being supplied with mains voltage via a switch device and its associated control unit in order to connect the mains voltage, when starting the oven, at a moment substantially coinciding with a mains voltage maximum.
In a microwave oven it is a requirement that the high voltage transformer shall be connected to the mains voltage at a desirable phase, coinciding with a voltage maximum of the mains voltage. By establishing the connection in this phase the connection current will be as low as possible and a strong current pulse on the mains is avoided, this being a requirement in several countries in order to allow a connection of the microwave oven to the mains. By connecting in this manner the sound effect otherwise appearing as a consequence of a strong connection current through the high voltage transformer is suppressed. Practically, this requirement means that the connection to the mains shall take place within a limited time interval around the mains voltage maximum.
In prior art microwave ovens a so called triac is used for this connection. The use of a triac has a number of drawbacks. A triac is sensitive to interferences, which may have the consequence that the connection takes place at a moment which differs from the desirable phase. A triac has a high heat dissipation at the power levels in question, meaning in turn a requirement of special cooling. The cooling is normally obtained by means of cooling plates, which must be relatively large and consequently space demanding. A further drawback is that the triac needs a special current supply in order to provide isolation between the power current part and the control system of the oven, this being a safety requirement in several countries. This may be obtained by the use of a so called opto-triac, an auxiliary winding of the transformer or a corresponding solution, meaning as a consequence increased complexity and increased costs.